In recent years, a semiconductor light-emitting device, such as a p-n junction light-emitting diode (LED) or a laser diode (LD), has been known as a compound semiconductor device. In the light-emitting device, when a group-III nitride semiconductor (AlxGayInzN: 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z=1) is used for a light-emitting layer, it is possible to obtain a light-emitting device that emits near ultraviolet light, blue light, or green light. In the group-III nitride semiconductor light-emitting device, generally, a substrate is formed of an electric insulating sapphire (α-Al2O3) single crystal. However, the substrate may be formed of a silicon carbide (SiC).
The compound semiconductor device is generally manufactured by forming a wafer having a plurality of compound semiconductor devices arranged in a predetermined pattern on a substrate and dividing the wafer into compound semiconductor devices. In general, a die saw or a scriber is used to divide the wafer. The die saw cuts the wafer by rotating a disk-shaped blade (dicing blade) having a diamond tip. The scriber forms very fine linear grooves (scribe lines) in a wafer, for example, in a lattice shape using a diamond stylus and applies external force to divide the wafer.
However, when a semiconductor light-emitting device having an epitaxial layer made of a group-III nitride semiconductor formed on a sapphire substrate is manufactured, a large distortion occurs in the interface between the group-III nitride semiconductor layer and the sapphire substrate due to a large difference between the lattice constants of the group-III nitride semiconductor layer and the sapphire substrate. In addition, since the sapphire substrate has a hexagonal crystal structure, it does not have a cleavage property. Both the sapphire substrate and the group-III nitride semiconductor have a Mohs hardness of about 9, which is a very high level. Therefore, it is difficult to divide the sapphire substrate having the group-III nitride semiconductor light-emitting device structure formed thereon using the scriber to manufacture semiconductor light-emitting devices having a regular shape with high yield. When the sapphire substrate having the group-III nitride semiconductor light-emitting device structure formed thereon is divided by the die saw, a crack or chipping is more likely to occur in a cutting plane, and it is difficult to clearly divide the substrate. As a result, the group-HI nitride semiconductor layer is likely to be peeled off from the sapphire substrate.
Further, a scribing method using a laser beam has been proposed (for example, see Patent Documents 1, 2, and 3). When a laser beam is radiated onto a wafer for a group-III nitride semiconductor light-emitting device to form division grooves in the wafer, it is possible to improve yield or productivity, and effectively and clearly form division grooves.
Furthermore, it has been known that a wafer having a gallium nitride compound semiconductor formed on a sapphire substrate is curved due to the difference between the lattice constants or the thermal expansion coefficients of the compound semiconductor and the sapphire substrate (for example, see Patent Document 4).
[Patent Document 1] Japanese Patent No. 3449201
[Patent Document 2] JP-A-11-177137
[Patent Document 3] JP-A-2006-24914
[Patent Document 4] JP-A-06-283758